Encapsulation of drugs into microparticles (e.g. nanoparticle and nanocapsule delivery systems) provides several advantages for in vivo drug delivery, including the ability to modify the drug""s biodistribution and to increase its bioavailability. These advantages are particularly important for therapeutic polar (i.e., water-soluble) macromolecules, such as polypeptides, polysaccharides, and polynucleotides, which otherwise have poor bioavailability, particularly when administered orally. However, prior to the present invention, satisfactory techniques for encapsulating water-soluble drugs into submicron particles were unknown.
One important feature of microparticles is the protection they afford to drugs from acid and enzymatic hydrolysis in the gut. For example, once delivered to a subject, submicron particles may be taken up via the gut-associated lymphoid tissue (GALT), commonly known as Peyer""s patches, into the lymphatic circulation. This route of uptake avoids hepatic first-pass metabolism and permits a therapeutic drug level to be achieved using a smaller dose, since metabolic systems need not be saturated. Moreover, intramuscularly or subcutaneously injected submicron particles are also capable of entering the lymphatic system and thus can circulate throughout the body. The material properties of the submicron particle wall or matrix also can be tailored to allow programmed release of the drug, thereby further improving the drug""s biodistribution and bioavailability.
The polymers used to form the matrices or capsule walls of microparticles are typically not water-soluble and therefore are not miscible with water-soluble drugs. Accordingly, the water/oil/water (w/o/w) double emulsion process has typically been used to encapsulate hydrophilic drug molecules into microparticles. This process involves the dispersion of an aqueous solution containing drug into an organic phase containing a preformed polymer in solution. The primary water/oil (w/o) emulsion is in turn dispersed into a second aqueous phase containing an emulsion stabilizer. This technique has been shown to efficiently encapsulate hydrophilic drugs, such as proteins, polynucleotides, or polysaccharides, with adequate core loads into particles larger than 1 xcexcm. However, particles greater than 1 xcexcm are taken up by the GALT far less efficiently than submicron particles. Therefore, the bioavailability of encapsulated molecules would be significantly higher if particles containing high drug loads could be manufactured in the submicron range.
Particle uptake via the GALT after oral delivery increases exponentially as particle size decreases from 5 xcexcm into the submicron range. Similarly, in cases where it is desired that subcutaneously or intramuscularly injected particles circulate in tissues, particle size must be less than approximately 5 xcexcm. However, the efficiency with which drug molecules can be encapsulated, particularly large, water-soluble molecules, decreases dramatically as particle diameter decreases below approximately 1 xcexcm. Therefore, creating submicron particles capable of being taken up efficiently by the GALT or capable of circulating within tissues that also contain sufficient drug content to allow therapeutic drug concentrations to be achieved is one of the main challenges in the pharmaceutical industry.
In a typical encapsulation involving the coarse water/oil/water double emulsion technique, the internal aqueous phase is usually dispersed into oil at a volume ratio of 1:2 to 1:20 (w:o), with higher encapsulation efficiencies observed for lower ratios of water to oil. Particles as small as 1 to 3 xcexcm in diameter may be generated using this technique. However, the size of the internal water droplets has a lower limit determined by the physical properties of the internal water and oil phases. The size of the internal water droplets in turn determines the efficiency with which drug may be encapsulated in particles in the submicron size range.
In addition, another problem associated with past techniques of forming submicron particles is that coarse emulsions typically formed to encapsulate water-soluble molecules in microparticles are not thermodynamically stable. Internal water droplets will tend to fuse and become larger if the particles are not quickly hardened. As one attempts to reduce the overall particle size, for example, by increasing mixing energy, and/or decreasing the viscosity of the primary emulsion, the encapsulation efficiency decreases due to increased opportunity for internal water droplets to diffuse to the outer surface of the oil phase and deposit the contents of the internal aqueous phase into the external aqueous medium. The end result of this thermodynamic instability of the internal w/o emulsion is that the proportion of drug associated with polymer becomes increasingly restricted to the surface of the particles which causes it to be quickly released (referred to as a xe2x80x9cburstxe2x80x9d) from the nanoparticle after dosing, a result which is often contrary to the intended release profile. In the case of oral delivery in particular, significant quantities of drug may be released before the particles are taken up across the gut mucosa.
Other strategies have been attempted to efficiently encapsulate water-soluble molecules into submicron particles. For example, naturally occurring hydrophilic polymers, such as albumin or gelatin, have been used to generate matrix-type nanoparticles. However, while hydrophilic polymers are compatible with water-soluble drugs and therefore have the potential for high loads and high encapsulation efficiencies, the hydrophilic surfaces of these particles are less likely to be taken up via the GALT than are particles of similar size with hydrophobic surfaces. Moreover, extensive processing is often required to remove toxic chemical crosslinking agents used to harden the particles. Heat denaturation has also been used to form hardened particles with the problem that heat often destroys the bioactivity of encapsulated drugs.
Methods for encapsulating drugs into microparticles composed of preformed polymers, such as the spontaneous emulsification process, have also been described (see e.g., U.S. Pat. No. 5,118,528). However, while submicron particles with uniform size distributions have been formed using this technique, it has been shown that large, water-soluble drugs are not efficiently encapsulated and high burst release characteristics are common (Niwa et al. (1994) J. Pharm. Sci. 83:727). Another drawback to the technique is that only limited volumes of aqueous drug solutions can be added to the polymer solution without affecting polymer solubility when hydrophobic polymers are used. Furthermore, low molecular weight polymers with a higher polar character than polylactic acid tend to precipitate without encapsulating, rather than form nanoparticles.
U.S. Pat. No. 5,049,322 describes a modified technique for the production of nanocapsules using preformed polymers. In this technique, an oil, a solid suspension, or volatile organic solution containing drug is dispersed into a water-miscible organic solvent, usually acetone, containing a solution of polymer. A polymer wall is deposited around solid particles or oil droplets when the oil phase is poured into a second continuous, usually aqueous phase that is a nonsolvent for the polymer. However, this patent describes a system for the encapsulation of material compatible with oils or organic solvents rather than aqueous solutions.
Other nanoparticle encapsulation techniques, such as the phase inversion method described in U.S. Pat. No. 6,143,211, require that the hydrophilic drug molecule be in a micronized, solid form and be suspended, rather than dissolved, in the organic phase. This has the disadvantage that dehydration of certain classes of water-soluble molecules, such as proteins, in addition to requiring expensive material processing steps, often results in irreversible structural and functional damage. Furthermore, dissolution of encapsulated material in tissue fluids may be incomplete, or, in the case of proteins, may form immunogenic aggregates.
Accordingly, improved techniques for efficiently encapsulating hydrophilic (water-soluble) drugs into microparticles, particularly submicron particles, at core loads that result in pharmacological activity, particularly after oral delivery, would be of great benefit.
The present invention provides an improved method and composition for encapsulating a wide variety of water-soluble agents into microparticles, including submicron particles (e.g., nanoparticles or particles having a size of less than about 1000 nanometers), capable of effectively delivering such agents in vivo, particularly when administered to subjects orally. The method involves forming a microemulsion containing an aqueous drug solution solubilized in oil, and subsequently encapsulating the microemulsion in a polymer shell. The resulting microparticles contain high drug core loads with minimal surface adsorbed drug, thereby reducing any burst effect.
While the method of the present invention has particular advantages for the encapsulation of water-soluble molecules into submicron particles, the method also can be used to form microparticles of larger size. This may be preferable, for example, in instances where delivery of water-soluble agents from a non-circulating injected depot is desired.
Accordingly, in one embodiment, the present invention provides a method for encapsulating a water-soluble agent by (a) forming a microemulsion comprising the agent; (b) adding the microemulsion to a first solvent comprising one or more polymers, thereby forming a dispersion; and then (c) adding the dispersion to a second solvent which is a nonsolvent for one or more polymers, resulting in encapsulation of the microemulsion by the one or more polymers in the form of microparticles.
In a particular embodiment, the drug-containing microemulsion formed in the invention comprises about 10% to 60% oil by volume. The microemulsion can further contain one or more surfactants or co-surfactants. Suitable surfactants include, but are not limited to polyoxyethylene sorbitan monooleate alone, sorbitan monolaurate, and mixtures thereof. Suitable co-surfactants include but are not limited to short to medium chain alkyl- or branched chain alcohols, such as ethanol, propanol, isopropanol, butanol, isobutanol, pentanol and isopentanol.
Typically, the microemulsion is added to the first solvent containing a polymer at a concentration of about 0.01% to 30% (w/w), more preferably about 0.1% to 10% (w/w). Suitable polymers include, for example, organic polymers such as polyvinyl alcohols, polyvinyl ethers, polyamides, polyvinyl esters, polyvinylpyrrolidone, polyglycolides, polyurethanes, alkyl celluloses, cellulose esters, hydroxypropyl derivatives of celluloses and cellulose esters, preformed polymers of poly alkyl acrylates, polyethylene, polystyrene, polyactic acid, polyglycolic acid, poly(lactide-co-glycolide), polycaprolactones, polybutyric acids, polyvaleric acid and copolymers thereof, alginates, chitosans, gelatin, albumin, zein and combinations thereof. A preferred polymer is poly(lactide-co-glycolide). Accordingly, the first solvent can be any suitable solvent for such polymers including, for example, ethyl acetate, benzyl alcohol and propylene carbonate.
In contrast to the first solvent, the second solvent is a nonsolvent for the selected polymer(s). A preferred second solvent is water. The second solvent can further include one or more emulsifying agents or surfactants to improve the formation of the microparticles upon addition of the microemulsion-containing dispersion.
Microparticles are then formed by adding the dispersion to the second solvent. The microparticles are typically in the submicron size range and are composed of a microemulsion containing a water-soluble agent encapsulated by one or more polymers.
A wide variety of water-soluble therapeutic agents, such as proteins, peptides, nucleic acids and other polar drugs, can be encapsulated using the method and composition of the present invention. Moreover, the resulting microparticles can be effectively administered to subjects using a variety of techniques, including parental (e.g., injection), topical and oral administration. Thus, the present invention is broadly applicable to many in vivo drug therapies.